Language Evolution and Computation Bibliography

This article reviews several studies on the behavioral and neuroanatomical asymmetries in non-human primates pertaining to the production of communicative behaviors. Hopkins and his team have reported population-level right-handedness for interspecies manual gestures associated with the request for food from a human. Meguerditchian and co-workers examined handedness for a variety of manual gestures during inter and intra-species communication in captive chimpanzees and found that the apes were significantly right-handed for all gesture types. Hauser reported that the left side of the face began to display facial expressions earlier than the right side for open-mouth threat and fear grimace in rhesus monkeys. Hauser and Akre examined the onset of mouth-opening asymmetries in rhesus monkeys during the production of several types of vocalizations. Hook-Costigan and Rogers showed that common marmosets displayed a larger left hemi-mouth during the production of fear expressions, including those that were or were not accompanied by a vocalization. Fernandez-Carriba with co-workers reported significant left orofacial asymmetries for several facial expressions including hooting, plays, silent-bared teeth, and scream face. Losin's team assessed orofacial asymmetries for four facial expressions associated with vocalizations in chimpanzees including hooting, food-barks, extended food grunts, and raspberries. Losin has found that food-barks and hoots were expressed more intensely on the left side of the face whereas extended food grunts and raspberries were expressed more intensely on the right side.

The evolutionary origin of human language and its neurobiological foundations has long been the object of intense scientific debate. Although a number of theories have been proposed, one particularly contentious model suggests that human language evolved from a manual gestural communication system in a common ape-human ancestor. Consistent with a gestural origins theory are data indicating that chimpanzees intentionally and referentially communicate via manual gestures, and the production of manual gestures, in conjunction with vocalizations, activates the chimpanzee Broca's area homologue a region in the human brain that is critical for the planning and execution of language. However, it is not known if this activity observed in the chimpanzee Broca's area is the result of the chimpanzees producing manual communicative gestures, communicative sounds, or both. This information is critical for evaluating the theory that human language evolved from a strictly manual gestural system. To this end, we used positron emission tomography (PET) to examine the neural metabolic activity in the chimpanzee brain. We collected PET data in 4 subjects, all of whom produced manual communicative gestures. However, 2 of these subjects also produced so-called attention-getting vocalizations directed towards a human experimenter. Interestingly, only the two subjects that produced these attention-getting sounds showed greater mean metabolic activity in the Broca's area homologue as compared to a baseline scan. The two subjects that did not produce attention-getting sounds did not. These data contradict an exclusive gestural origins theory for they suggest that it is vocal signaling that selectively activates the Broca's area homologue in chimpanzees. In other words, the activity observed in the Broca's area homologue reflects the production of vocal signals by the chimpanzees, suggesting thast this critical human language region was involved in vocal signaling in the common ancestor of both modern humans and chimpanzees.